1. Field of the Invention
This invention relates to automotive braking systems and in particular to spot-type automotive disc brakes incorporating a secondary or parking brake function.
2. Description of the Related Art
The state of development of spot-type automotive disc brakes is such that it is indisputable that the absence of a simple and successful and cost-effective parking brake function comes as a surprise and is indicative of serious technical difficulties in providing this fundamental requirement for automotive vehicles, bearing in mind the considerable investment of the automotive industry in design and development of disc brake systems over four or five or more decades.
Automotive vehicles incorporating disc brake systems are of course provided with parking and secondary brake systems which are in use, and these will be reviewed below together with the most pertinent ones of the paper proposals which are also known to the applicants. So far as parking brake systems in actual use are concerned, the position is that (with one exception, see below) there is not in actual commercial use, to the best of the Applicants"" knowledge, a spot type automotive disc brake in which a parking or secondary brake function is provided using the same friction elements as are actuated by the primary braking system. Usually of course the primary braking system is a hydraulic system. The nearest approach to such a dual purpose disc brake is provided on certain models of Jaguar cars and utilises an additional pair of friction elements and an associated additional mechanical actuating mechanism for these, so that the primary hydraulic disc brake system is provided with a secondary mechanical disc brake offering the parking brake function. Such an arrangement is an undesirable response to the need for a simple and economical disc brake system which would be applicable to small inexpensive vehicles as well as to larger and more costly ones.
All the other parking brake systems currently used in relation to automotive vehicles having disc brakes involve some use of drum brakes, such as the provision of rear drums (in substitution for the rear disc), or the provision of rear drums as a supplement to the rear disc brakes, or the provision of a drum-type transmission brake. None of these proposals, likewise, meets the requirement for a simple and cost-effective and compact disc brake system providing both primary and secondary/parking brake functions.
So far as paper proposals are concerned, the applicants are aware of:
U.S. Pat. No. 4,499,977 (Wang)
U.S. Pat. No. 3,941,221 (Pringle)
U.S. Pat. No. 3,724,605 (Naismith)
and these prior proposals will be reviewed.
The Wang specification discloses an integral park brake mechanism for a disc brake of the fixed disc and moving caliper kind in which the usual hydraulic piston and cylinder assembly 1, 15 in the caliper on one side of the disc acts through a rod 2 so as to actuate the friction element at that side of the disc (and through the moving caliper the other friction element is also actuated). The parking brake function is integrated with the hydraulic actuating function by means of an apertured locking collar 3 through which the rod 2 extends and which can be tilted to grip the rod and to apply to the rod an actuating thrust from a parking brake lever 4.
This arrangement is stated to meet the requirement for locking during actuation and self-releasing after actuation. However, the Applicants know that such an arrangement is technically undesirable for actual commercial use due to the obvious fact that a simple locking mechanism involving a rod and a locking collar, with the rod sliding in a bore formed in the hydraulic piston, is completely undesirable in terms of mechanical reliability for a mechanism located in the hostile environment of automotive disc brakes where temperature extremes and significant amounts of water and dust and other foreign matter are routinely applied to the mechanism during routine daily use. As a result, the mechanism is obviously unreliable and the statements in the description at column 4 line 25 onwards that the operation of the park brake mechanism is quite independent of the operation of the hydraulic brake, and the reference to the failure of one of the brakes not affecting the function of the other, is clearly technically wrong. Obviously a locking mechanism of the kind disclosed is liable to stay locked after use in the conditions described. Likewise the free sliding of the rod 2 within the bore in the hydraulic piston is likely to become restricted and finally prevented so that the hydraulic and parking brake functions are locked together and rendered unreliable if not inoperative. Hence the reasons why this mechanism has never been introduced in actual commercial use.
The Pringle brake takes a different approach to the construction of the parking brake mechanism by adopting a mechanical actuator located within the piston and cylinder assembly of the primary brake actuator system. This is also the xe2x80x9cone exceptionxe2x80x9d mentioned earlier and a fixed disc disc brake having such a parking brake (but otherwise not constructed as disclosed in Pringle) is in commercial use. Such an arrangement is mechanically difficult and costly to construct. This is partly because the mechanism has to be constructed relatively so small due to the space constraints imposed by the correspondingly small piston and cylinder assemblies utilised in disc brakes for the rear wheels of small vehicles, taking account of the front/rear braking proportions which are conventionally utilised. As a result, the mechanical parking brake mechanism within the hydraulic cylinder is effectively a mechanism almost of watch-like construction complexity and proportions and which is required to operate in the hostile environment of a braking system in terms of the temperatures and hydraulic fluid. In short, such an arrangement is technically and commercially undesirable as a solution to the requirement for a simple and cost effective mechanism. These same comments apply equally to the Naismith disclosure which shows a similar mechanism.
Two additional prior art references which have come to the attention of the applicants are:
GB2177171A (Kelsey Hayes)
WO98/05879 (Brake Technologies Pty)
The Kelsey-Hayes specification discloses improvements in self-adjusting parking brakes, particularly for use on rear-axle disc brake assemblies. The parking brake actuator is preferably a xe2x80x9cbolt-onxe2x80x9d type assembly intended for external mounting to a disc brake caliper. Use of a xe2x80x9cbolt-onxe2x80x9d parking-brake actuating assembly eliminates the necessity of having front and rear axle disc brake assemblies of different design since the parking brake mechanism is not an integral part of the disc brake assembly (page 1, column 1, at lines 35 to 40). Therefore, as shown in FIG. 3, the parking brake arrangement is asymmetrically arranged with respect to the main structure of the brake, and such is unacceptable in terms of its effect on the operation of the primary braking mechanism due to the need for balanced and uniform operating and wear characteristics, as far as possible. The deliberate adoption of an asymmetric configuration causing corresponding non-uniformity of operating and wear characteristics is generally unacceptable.
In the Brake Technologies Pty specification, there is disclosed a disc brake assembly of the kind in which the brake disc 23 is oil-immersed, and provided with full-annulus (or substantially full-annulus) friction element engagement facilities. Brakes of this kind are conventionally used as transmission brakes in heavy duty vehicular applications, including tractor brakes, for example. A primary actuating mechanism of the hydraulic kind is provided, together with a fail-safe emergency/parking type braking mechanism based upon spring elements 62, 63 to cause application of the emergency-parking brake in the event of failure of the hydraulic fluid system. The general arrangement of the brake can be seen in FIGS. 1 and 2, and it is evident that such a brake is not of the spot-type automotive disc brake kind employing a lever-operated parking brake mechanism. Therefore, taking account both of disc brake systems in use and prior proposals, there remains a substantial requirement for a lever-operated parking and/or secondary brake system for use in spot-type automotive disc brakes and meeting one or more of the requirements outlined above, or at least providing a better compromise between the various conflicting factors than the disclosures in the prior proposals discussed above.
Thus, we have identified a need for the provision of a parking brake assembly in which the complexity and cost and unreliability of the Naismith and Wang disclosures is mitigated or overcome, while preserving the lack of asymmetry of the actuating arrangements therein. Likewise, we have identified the need for eliminating the asymmetry which is fundamental to the approach in the Kelsey-Hayes disclosure and for eliminating the duplication of hardware likewise inherent in the Jaguar construction. Thus the embodiments of the present invention seek to provide, particularly for the cost-conscious spot-type automotive disc brake market, a parking or secondary brake assembly which is able to use the simple thrust-generating capabilities of a lever mechanism while avoiding duplication of actuation systems and friction pads, and avoiding likewise the actuation asymmetry which has represented the Achilles heel of the other proposals discussed above.
In the described embodiments of the invention, there is provided a parking brake arrangement in which, by adopting a lever mechanism which is constructed and adapted in accordance with the format of the primary brake operating mechanism (primarily by symmetrically accommodating either a single central cylinder or two spaced actuating cylinders), the combination of the convenient and effective generation of actuation thrust by the lever mechanism, and a simple symmetrical thrust-application system in which the friction elements have a reasonable chance to wear uniformly.
By adopting a lever construction which can straddle a single actuating cylinder, or the equivalent arrangement in which twin actuating cylinders straddle a single lever assembly, the problem which led the prior art to adopt mechanisms of watch-like complexity and/or an out-of-balance asymmetry, is solved.
Therefore, taking account both of disc brake systems in use and prior paper proposals, there remains a substantial requirement for a parking and/or secondary brake system for use in automotive disc brakes and meeting one or more of the requirements outlined above, or at least providing a better compromise between the various conflicting factors than do the prior proposals discussed above.
According to the present invention there is provided a disc brake as defined in the accompanying claims.
In an embodiment of the invention described below the primary and secondary actuating mechanisms of the brake are constructed so as to be completely independent with respect to each other. As a result, the thrust applied by each actuating mechanism to the same one of the friction elements (on that side of the disc) reaches that friction element by a path which is independent and separate from that of the other mechanism. As a result, there is no common thrust transmission component in the primary and secondary actuating mechanism (as is the case in the Wang and Naismith references). Each of these mechanisms acts on the same friction element, but in fact applies its thrust to that friction element through an end thrust delivering surface which is spaced from the corresponding surface of the other actuating mechanism. In other words, the two mechanisms act on the friction element at laterally spaced-apart locations. In one embodiment, the piston of the primary hydraulic actuating mechanism acts on the friction element though a part-circular or cylindrical projecting structure integral with the piston, and the thrust delivering end of the parking brake mechanism is received (with clearance) in a slot formed therein and engages the friction element generally on the axis of and thus symmetrically with respect to the piston and through the curved surface of a profiled thrust-applying member.
In an alternative arrangement in which the primary actuating mechanism has spaced apart twin cylinders, the secondary or parking brake mechanism is disposed symmetrically between these.
Other aspects of the independent relationship of the primary and secondary actuating mechanisms include the following. Firstly, a malfunction of one mechanism has no effect on the other mechanism whereas, for example, in the Wang mechanism the adoption of thrust paths for the two mechanisms which coincide at the rod 2 of Wang means that a failure of one mechanism is likely to seriously affect the other mechanism. Likewise, the complete independence of the primary and secondary actuating mechanisms in the embodiments of the present invention also means that the parking brake mechanism imposes no constraints on the retraction of the primary or hydraulic system after use, such as would occur in Wang when the rod 2 no longer slides freely within the bore of the piston 1. Moreover, in specific contradistinction to Wang, the piston of the hydraulic mechanism in the embodiments of the present invention provides no mounting whatever for the parking brake mechanism. The piston of the embodiments serves only to generate brake-actuating thrust. According to another important feature of the embodiments of the present invention, the rotatable disc is mounted so as to be capable of sliding movement axially thereof and said friction elements being mountable on a fixed caliper or bridge structure straddling the disc, whereby the brake-applying thrust applied by the secondary actuating mechanism to provide a secondary or parking brake function causes frictional engagement of the disc with the other one of the pair of friction elements without the need to transmit a thrust to that friction element through a moving caliper or bridge structure, and the reaction thrust generated by both said primary and secondary actuating mechanisms is resisted by said fixed caliper or bridge structure. In other words, by virtue of adopting a sliding disc configuration for the disc brake, the parking brake function can likewise adopt a simple one-sided configuration without the need (such as arises in a fixed disc brake assembly) to provide a corresponding oppositely-directed thrust to the friction element on the other side of the disc. This latter friction element is simply squeezed against a fixed stop (provided by the fixed caliper or bridge) by the axial movement of the disc under the action of the parking brake lever mechanism, and the reaction thrusts are all resisted well.
To put it another way, the general configuration of the disc brake utilising a sliding disc and a fixed caliper in the embodiments means not only that the one-sided lever action is sufficient to apply both friction elements to the disc, but also the general structure of the brake is such that the reaction forces generated by the lever mechanism can be conveniently applied to the robust and fixed and stable structure which supports the fixed caliper or bridge of the brake and in no way compromises the effective operation of the primary braking system.
In order to create a well-balanced actuation arrangement for the brake, the embodiments provide the secondary brake actuating mechanism comprising a bifurcated lever member which straddles the piston and cylinder assembly of the primary actuating mechanism so as to be able to apply a balanced and symmetrically distributed thrust therewith to the friction element generally on the axis of the piston. Where the brake comprises a pair (or more) of piston and cylinder assemblies, then the lever mechanism can be disposed symmetrically between these without the need actually to invade the central space of the piston (by means of a slot etc therein) as described above.
To compensate for wear of the friction elements, the secondary brake actuating mechanism is provided with an adjustment mechanism. In one embodiment, the adjustment mechanism is adapted to move the pivot of the lever mechanism towards the actuated friction element as wear of the friction element occurs, whereby the lever is maintained in a constant actuating attitude relative to the friction element despite the wear of the latter.
In an alternative adjustment arrangement the lever of the lever mechanism is provided with an adjustment member which progressively moves as the friction element wears so as to change the dimensions of the lever member accordingly. In one arrangement, this is achieved by a pivoted sub-lever mounted on the main lever and controlled by a simple ratchet mechanism.
Further embodiments of the invention provide two or more slidable discs with interleaved axially movable friction elements between the discs. The brake may be provided as a front (or even intermediate) wheel of a vehicle as well as or alternatively to the use of the brake on a rear wheel of a vehicle.